The present invention relates to bulk acoustic resonator devices, more particularly to tuning thin film resonator filters.
Bulk acoustic wave devices such as thin film resonators (hereinafter xe2x80x9cTFRxe2x80x9d) are typically used in high-frequency frequency control and filtering applications ranging from several hundred megahertz (MHz) to several gigahertz (GHz). A TFR typically is comprised of a piezoelectric material interposed between two conductive electrodes, one of which can be formed on a support structure. The support structure can be a membrane formed by removal of material beneath it, or a plurality of alternating acoustic reflecting layers formed on a semiconductor substrate such as silicon or quartz, for example. The piezoelectric material is typically AIN, but may also be formed of ZnO or CdS amongst other piezoelectric material. The electrodes are formed from a conductive material, preferably of Al, but may be formed from other conductors as well. These films are deposited and lithographically patterned into their useful form in much the same way modern integrated circuits are made.
TFRs are often used in electronic signal filters, more particularly in TFR filter circuits applicable to a myriad of communication technologies. For example, TFR filter circuits may be employed in cellular, wireless and fiber-optic communications, as well as in computer or computer-related information-exchange or information-sharing systems.
The desire to render these increasingly complicated communication systems portable, even hand-held, places significant demands on filtering technology, particularly in the context of the increasingly crowded radio frequency spectrum. TFR filters must meet strict physical requirements which include: (a) being extremely robust, (b) being readily mass-produced and (c) being small while maintaining the required strict rejection and transmission characteristics. Restated, there is a simultaneous need for low passband insertion loss and for a large stopband attenuation in order to effectively clean up, for example, signals at the front-end of an RF radio. Some cellular phone applications for these TFR filters require passband widths up to 4% of the center frequency (for example, for a 2 GHz center frequency, this would be a bandwidth of about 80 MHz). This is not easily accomplished using common piezoelectrics such as AIN, and careful design and manufacture steps must be taken to keep filter bandwidths as wide as possible.
The piezoelectric material in TFR resonators converts electrical to mechanical energy and vice versa such that at its mechanical resonance frequency, the electrical behavior of the device abruptly changes. Electrical signals of particular frequencies easily pass thorough the resonators, while others will not be transmitted. These particular frequencies can be dictated by choosing resonator size and design. Resonators of certain sizes and design frequencies can be networked in appropriate combinations, such that they will impose desired filtering functions on signals passing through the network. A standard approach to designing filters out of resonators is to arrange them in a ladder configuration alternately in a series-shunt relationship. A series element in this sense carries signal from an input toward an output, whereas a shunt element provides an alternative path for the signal to ground. The transmission or blocking characteristics of both series and shunt elements affect the final signal reaching output from input, somewhat analogous to how branching of water pipes can affect the flow through the main water line.
Currently, the conventional way of designing TFR ladder filters is to design simple building blocks of TFR components having moderate selectivity, and then to concatenate these building blocks together (connected or linked up in a series or chain) to obtain a stronger filtering characteristic. In a simplified view, concatenation helps to achieve a larger stopband attenuation for the filter because each individual linked up section in the chain successively filters the signal more as it passes through the chain.
To make wide bandwidth filters from piezoelectric resonators, it is known that resonators of at least two differing frequencies are required. The difference in the frequencies will be similar to the required filter bandwidth. Numerous strategies are employed depending whether bandpass, bandstop, or any number of other filter shapes is required. Designs can be complicated and require more than a simple pair of frequencies. We shall illustrate an advantageous way to produce, in a batch fabricated manner similar to making integrated circuits, resonators on a single substrate of differing frequencies for use in any number of filtering applications. We shall describe the technique in the light of making a bandpass filter, but it will be realized the technique is applicable to making any number of filters requiring a multiplicity of differing frequency resonators.
FIG. 1 illustrates schematically illustrates this simple building block, commonly known as a T-Cell. Referring specifically to FIG. 1, a schematic of a T-Cell building block 100 includes three TFR components 110, 120 and 130. TFR components 110 and 120 comprise the xe2x80x9cseries armxe2x80x9d portion of the T-Cell block, being connected in series between an input port 115 and an output port 125 of T-Cell 100. TFR component 130 comprises the xe2x80x9cshunt legxe2x80x9d portion of T-Cell 100, being connected in shunt between node 135 and ground. A TFR T-Cell itself may define a filter; although a TFR ladder filter typically has a plurality of these T-cells concatenated together.
FIGS. 2A-2C graphically illustrate how a bandpass filter response for bulk acoustic wave devices such as resonator filters are conventionally achieved. Each of the shunt and series TFR components 110, 120 and 130 in the schematic T-Cell of FIG. 1 has a set of characteristic frequencies: a xe2x80x9cpolexe2x80x9d frequency and a xe2x80x9czeroxe2x80x9d frequency. The terms refer to the magnitude of the impedance to current flow through the device; impedance is low at the zero and high at the pole. The series and shunt arms in a filter typically have zero and pole frequencies slightly shifted from each other. As will be explained further below, the current method of achieving an acceptable bandpass filter response has been to shift the frequencies of the shunt TFR component down in frequency.
Providing resonator components having desirable impedance characteristics is a necessary requirement for building a TFR-based filter. FIG. 2A illustrates a typical transmission response for a series TFR component of a TFR filter. Referring to FIG. 2A, a single, series-wired TFR component will have the voltage transmission response S21 (as shown in FIG. 2A, signal magnitude (y-axis in dB) as a function of frequency (z-axis GHz) shown at its output. FIG. 2A illustrates the following characteristics: the signal maximum (nearest the vertical zero, greatest transmission) occurs at about 1.90 GHz. This point is known as the resonator zero because of the nearly zero impedance to current flow. The point of least transmission is at about 1.94 GHz; this is the resonator""s pole, where it has the highest impedance to the flow of electrical current. FIG. 2A illustrates the behavior of a device whose transmission of an electrical signal varies as a function of the frequency which is the basic definition of a filter. However, this single TFR component by itself does not have the characteristics desired in typical filters, like high rejection away from the pass band, or a flat pass band in which transmission is uniform.
FIG. 2B illustrates a typical response for a shunt TFR component of a TFR filter. The difference between FIGS. 2A and 2B is that in FIG. 2A, the signal moving from input to output must flow through the TFR, whereas in FIG. 2B, any signal flowing through the shunt TFR will not reach the output since it shunts to ground. Referring to FIG. 2B, a circuit executed in this manner has a minimum transmission at about 1.90 GHz, since the signal passes through the shunt resonator (at its frequency of lowest impedance) instead of proceeding to the output. At the pole frequency of the shunt TFR component, very little of the signal goes through the TFR (since it is at its frequency of highest impedance.) Consequently most of the signal is transmitted from input to output.
The T-cell structure of FIG. 1 is a combination of TFR resonators in series and shunt to form the T shape, thus the name. A resulting bandpass filter is formed where signals with frequencies away from the band are blocked, and signals in the band are passed. The TFR series and shunt behavior discussed in the explanations of FIGS. 2A and 2B can be used to make a bandpass filter. It is common in filter design to shift the shunt element""s pole frequency to fall near the zero frequency of the series element so as to obtain near-uniform transmission in the center of the band. The resulting transmission behavior as illustrated in FIG. 2C
FIG. 3 illustrates a cross-sectional view of a typical TFR component, which is comprised of a layer of piezoelectric material 210 interposed between top and bottom metal electrodes 205 and 215 on a substrate 220. The piezoelectric material 210 is preferably AIN, but may also be ZnO, or CdS amongst other materials. The metal electrodes 205 and 215 may be thin metal films of Al or other conductors. The substrate 220 may consist of a plurality of reflecting layers mounted on a silicon wafer, or may be formed as a membrane. What is fundamentally required from the reflecting layers or air interface of a membrane structure is to have a good reflection of acoustic energy created in the piezoelectric material, such that this energy does not leak out of the resonator, ultimately causing an undesired loss of signal.
The mechanical resonance frequency of a TFR resonator is determined by the time it takes the acoustic wave to make a trip from the top surface to the bottom, undergo a reflection, and return to the top. The thinner the device, the faster the wave returns. In a simplified view, the resonance, or sympathetic vibration, occurs at the frequency where a wave being input into the device constructively adds to the wave introduced in the previous cycle, but which has now returned to its original location. Thus the resonance frequency of the TFR is set by the thickness and properties (i.e., speed, density) of the films deposited. To create a bandpass filter which exhibits a response like in FIG. 2C from such resonators, the shunt and series TFRs are manufactured so as to resonate at different frequencies (typically but not necessarily all the series TFRs at one frequency, all the shunt TFRs at another). This is typically done by fabricating the shunt and series TFRs with different thicknesses, or more particularly by increasing the thickness of the shunt TFR. Conventionally, the shunt resonator""s resonant frequency set (pole and zero) are reduced by adding a greater thickness of material to its top electrode. As shown in FIG. 3, for example, materials are added to the TFR component 200, such as a thin metal layer 216 added to top electrode 205 to reduce the resonant frequencies (pole and zero) of the TFR to be used as the shunt element of a filter.
However, the conventional method of adding a metal layer to the shunt electrode has some disadvantages. For example, added material which is not piezoelectric can detrimentally reduce the separation of the pole and zero frequencies; this may ultimately limit the maximum bandwidth of filters made from these structures. Also, metals are known to attenuate acoustic waves more than the insulating piezoelectric material, so it is desirable to minimize the fractional amount of metal in a resonator. Accordingly, what is needed is an alternative method of tuning a TFR filter that does not introduce these disadvantages.
The present invention provides a method for producing thin film resonator (TFR) filters formed from a plurality of TFR components coupled in series and shunt branches. Each of the plurality of TFR components has a set of resonant frequencies. A TFR bandpass filter can be produced by up-shifting the set of resonant frequencies in the series branch TFR components until the desired band shape is achieved. For example, this may be accomplished by removing material from the series branch TFR component, rather than by adding material to downshift the frequency of the shunt TFR components. Additionally, a TFR bandpass filter may be produced by down-shifting the set of resonant frequencies in the shunt branch TFR components until the desired band shape is achieved. Further, a TFR bandstop filter can be produced by up-shifting the set of resonant frequencies in the shunt branch TFR components until the desired stop band response is achieved. This can be accomplished by removing material from the shunt branch TFR components.